Mix the problems: interleaved practice for study

Part 1 of 5, · The technique

Mix the problems

Chapter 5 ends with 20 questions on one method. By the third, you’re on autopilot: you know the method before you read the question. Then the test mixes chapters 3 to 5. Now the hard part is knowing which method each needs.

Interleaving, or mixed practice, shuffles different kinds of problem into one set, instead of a block of each. Each question then asks: what kind is this, and how do I solve it?

What the research found

In a 2014 class study by Rohrer, Dedrick and Burgess, 140 Year 7 students did 9 weeks of maths practice, mixed or in blocks. On a surprise test 2 weeks later, material practised mixed scored 72% on average and blocked material 38%.

In 2021, Samani and Pan mixed the homework of 350 students taking introductory physics at university for 4 of 8 weeks. They got fewer mixed problems right and tended to think they learned less.

Yet on surprise tests of new, harder problems, held every 4 weeks, median scores after mixed weeks were 50% and 125% higher. On graded exams 3 days later, after most had crammed, there was no clear difference.

Mixing also spaces each kind out. Is the gain only spacing? In 2010, Taylor and Rohrer kept the spacing equal as children practised four kinds of maths problems. Mixing lowered practice scores but doubled test scores a day later. The errors suggested it helped them match each problem to its method.

A 2019 meta-analysis of 59 studies by Brunmair and Richter found a moderate benefit on average, small for maths and unclear for reading texts. For words, blocking did better. It helped more when the kinds were more alike.

Most studies of mixing examples of concepts are lab studies of university students, a 2021 review by Firth and colleagues found. In 2013, Dunlosky and colleagues rated mixing only moderately useful, as evidence was still limited.

Does it work with ADHD?

We found no study of mixed practice in people with ADHD. The closest is a small 2026 study in Spain by Arriola and colleagues. In it, 88 children and teenagers aged 8 to 16 counted flowers on a screen. Half had ADHD. Two near-identical kinds of flower came mixed or in blocks. Mixing helped both groups tell the kinds apart.

That’s noticing pictures, not studying. The rest is research with people in general, so treat mixing as a good bet, not a proven fix. Our view, not a finding: a block slides into autopilot, where drifting off is easy. A mixed set makes every problem a small puzzle.

How to do it

• Pick three kinds of problem you’ve learned in one subject.
• Shuffle them so no two of a kind sit together.
• Name each one’s kind and method, then solve it.
• Expect more mistakes than in a block.

The class studies kept the mix inside one subject. Mixing makes practice harder, but choosing the method is the practice. Give each subject a run of its own.

Part 2 of 5

Try this week

Takes about 20 minutes.

Choose one subject with three kinds of problem you’ve already learned, such as chapters 3 to 5 of maths. Take 5 minutes to pick two problems of each kind and write all six on one sheet, so no two of a kind sit together. Then work through them in order in one 15-minute run.

If it feels harder than a block, that’s expected. The physics students above rated it harder too.

Part 3 of 5, · On the timer

One mixed set, one named run

The Subject row and the What are you working on? box need an account and the full layout, the default (Settings → What’s on screen → Layout → Full). Signed out, start at step 3.

1. Under Subject, pick the subject. To add one, press + new, type its name and press Enter.
2. In What are you working on?, type the set’s name, for example ‘Mixed set: chapters 3 to 5’.
3. Under Length, choose 15m (not the 15m beside Break).
4. Press Start focus and work down the sheet in order. A short chime marks a full run.
5. If your mind wanders off, press I got distracted. That stretch is logged and the clock stops.
6. When you’re back, press Start focus again.

Another set, or another subject, gets a run of its own. If you finish a set before the chime, press End session first. Between runs, press 5m beside Break for a rest. Then pick the next subject under Subject, change the words in What are you working on? to the new run’s name and press Start focus.

Afterwards, click today’s bar on the chart. Signed in, Runs on … lists every run with its length, how it ended and what it was for.

Open the timer to try it